The question of which fuse or relay is the one for the glow plugs won't go away. Perhaps this one thread will eliminate the 40 or 50 each year that ask the same question over and over.
The fuse for the four cylinder glow plugs is located inside the cabin above the fuse and relay mounting block. Please see the images below for the locations on the B4 Passat (near the top of the inside firewall), and on the A3 Jetta (at the top of the fuse and relay block).

1996 B4
On the North American market 1996 Passat the glow plug relay is the one in position #12 of the fuse and relay block just above the driver's feet. The relay is identified by a "103" printed on its top. Note: The Bentley manual electrical schematic has a typo which indicates this glow plug relay is marked with "104". The image below shows a typical 1996 B4 relay layout. The glow plug fuse location is similar to that shown in the 1997 B4 image to follow.
This wiring harness requires the BK or GQ ECU.

1997 B4
On the North American market 1997 Passat the glow plug relay is moved to a remote location above the main fuse and relay block. There were several other wiring changes made at this transition. The glow plugs relays are not interchangeable between the early and the late wiring harness changes. The glow plug relay is now marked "180". A 'jumper relay' is installed in position #12 to allow electrical power to feed to the remote 180 glow plug relay location. The other relays in the relay and fuse block are positioned similar to those shown in the previous 1996 image. The image below shows the mounting location of the "180" glow plug relay in the 1997 and also the glow plug fuse location in both the 1996 and 1997 B4. Thanks to TonyJetta for the edit of PaulB's picture showing the 1997 relay and fuse location.
This wiring harness requires the FA ECU.

Important Note: The wiring harness change in the B4 TDI took place part way through the 1997 model year. According to the Bentley manual the change over took place at the end of October 1996 for the last of the early, BK/GQ/103 wiring harness, with a November 1996 start for the later, FA/180 wiring harness. Very early 1997 models, those built in June through October of 1996, might therefore have a harness similar to the 1996 models and would require the "103" relay.
The glow plug relays are not interchangeable between harnesses. The relays and/or the wiring harnesses can be damaged, and wiring harness fire inside the cabin is a real possibility, due to the dead shorts created by improper glow plug relay use.

1998 A3
The North American market 1998 A3 has the "180" glow plug relay attached above the main relay block. The glow plug fuse is mounted just above the glow plug relay, not separately on the inner firewall as on the B4. Also note in the image below that there is nothing in the lower right relay socket (location #12). This is a difference from the 97 B4 TDI which needs a jumper to feed power to the glow relay 180.
Note: The installation of a #12 jumper relay from a 1997 B4, or the installation of a "103" or "180" glow plug relay in this vacant position with the 1998 A3 wiring harness can also cause damage to electrical components and possibly fire from dead shorts.

Note: The Bentley wiring schematics show a change in the wiring of the glow plug relay during the A3 TDI production years. I have no further knowledge of the differences than what is described below in post #7.

__________________If the quantity of carbonic acid increases in geometric progression,
the augmentation of the temperature will increase nearly in arithmetic progression.
Svante Arrhenius 1896

Cogito ergo soy (I think, therefore: Biodiesel)

Last edited by Lug_Nut; November 22nd, 2009 at 14:20.
Reason: information re-arranged for better clarity

A word of caution when ordering a glow plug relay for a '96 or '97 B4...

A random sample of 4 retailers (Impex, Arizona Autohaus, BleachedBora & Adirondack Auto Parts) revealed glow plug relay listings to be incorrect. Either only one relay was listed for both years (Impex, BleachedBora) or two were listed but with incorrect transition production dates (Arizona Autohaus, Adirondack). If you can't use the supplier's p/n to determine if you're ordering the right one, use price. The '96 #103, p/n 357-911-253, relay is around $15, the '97 #180, 038-911-253, is around $50.

After digging out the GP Relay on my 97, I thought of a much quicker and less painful way to check if the relay is passing sufficient current. It tests how well the wiring and harness are working while in position too.

With your voltmeter, check the open circuit voltage at the GP connectors, with the temp sensor disconnected and the ignition on (as per the test process). If its 0, then you need to check the GP fuse and if its not the fuse then you're probably in need of a relay or some wiring.

Assuming you had 12v +/- at this first test, put the GP harness back on the GPs. Now take a straight pin or something similar and poke it into one of the main GP wires, either through the insulation or at a splice end if your harness is a replacement. Do the same test again, with the volt meter probe on this pin. BE CAREFUL that the pin does not touch any metal parts.

You should not see 12v again, if you do then either your GPs are not carrying current, or the connections to them through the harness are bad. If you see less than 8-9v or so, your relay and/or wiring to the GPs cannot supply enough current to the GPs (or your battery is weak). Something around 10v is about right. It shows that the GPs are drawing enough current to lower the voltage a bit, which is what you want. If you have a two wire harness, you may want to check both wires.

If you poked a hole in the insulation, be sure to cover it with electrical tape when you are done with this test.

Perhaps some of the EE's on the board can comment, but from my rudimentary knowledge of electricity, I think a test like this is a better way to check out the GP system, and a heck of alot easier than disassembling the dash as you need to do on the 97.

And if anyone has a good clear image of a 97 B4TDI, or of a 97 A3TDI main relay and fuse block, please post it. It'd be nice to see the extra relay socket #12 jumper for the 97 B4 and if the 97 A3 uses the same arrangement, or if the 97 A3 has an empty socket like the 98 A3.

__________________If the quantity of carbonic acid increases in geometric progression,
the augmentation of the temperature will increase nearly in arithmetic progression.
Svante Arrhenius 1896

96 B4
The 96's 50 amp glow plug fuse has no voltage when the glow plug relay is off. Fuse #18, 20 amp, can be used as a test point for glow plug relay function. Fuse #18 is powered on the same circuit as the glow plugs and will have voltage when the glow plug relay is on (contacts are closed).
Note that the Bentley B4 service manual's electrical schematic is in error on sheet X171 where it identifies the 96 glow plug relay with production number "104". The correct production number for the 96 B4 TDI is 103.

97 B4
The jumper in relay socket #12 of the 97 B4 TDI bridges socket ports 4 and 6 together to feed battery hot power to the glow plug fuse. The glow plug 50 amp fuse supply is always hot. The remotely mounted glow relay is after the fuse. Fuse #18, 20 amp, is therefore also always hot and can be used as a test point for socket 12 jumper integrity.

97 A3
In the 97 A3 the relay socket #12 is empty. The glow plug fuse is after the glow plug relay. The fuse is hot only when the glow relay contacts are closed. There is no easily accessible test point for confirmation of either one's condition.

98~99 A3
In the 98 A3 the socket #12 is empty. The glow plug fuse is before the glow plug relay. The fuse is always hot. Again there is no easily accessed test point for either.
Note that the Bentley A3 service manual's electric schematic, sheet 97-415, indicates the glow plug wires from the glow relay are 0.5 mm in cross section. The correct size is 5.0 mm cross section.

__________________If the quantity of carbonic acid increases in geometric progression,
the augmentation of the temperature will increase nearly in arithmetic progression.
Svante Arrhenius 1896

Cogito ergo soy (I think, therefore: Biodiesel)

Last edited by Lug_Nut; March 3rd, 2008 at 16:42.
Reason: clarifications

Lug Nut, can you comment on any variations in GP relay locations for Canadian-supplied TDI's? (maybe they are not different...?)
A good friend of mine "lost" his TDI to a fat-fingered mechanic here in South Carolina who decided to plug a "180" into position #12 on his 1999 Jetta. it's too late for the car, but someone else might have a Jetta from "up-Nawth" and run across this same problem.

Mine was similarly 'serviced' with a duplicate glow relay in #12 when I rescued it from the prior owner's mechanic. There was no apparent damage from this period.
There was some wiring insulation melted through and shorted wires, but once those were cut out and replaced, and the offending relay removed, the starter no longer spun up and engaged each time the clutch was pressed to the floor to shift.

__________________If the quantity of carbonic acid increases in geometric progression,
the augmentation of the temperature will increase nearly in arithmetic progression.
Svante Arrhenius 1896

"Friend's" computer got toasted.
Shop offered him the blue-book $...the car had over 150K miles on it (and got 52 mpg+ on the road.)
No-one around here uses this shop any more...
Their excuse was "Oh, it must have come from CANADA" (!) since he originally bought it in Maine.
There is no end of ways to screw-up something when you don't know what you are doing!!

I believe the best and easiest way to check glow plugs is with an inductive pick for current draw on the glow plug harness. You have to unplug nothing and clearly shows what the glow plugs are doing. Each glow plug will draw about 7 to 9 amps. So if you add it up you have total current draw.

Can someone describe how to gain access to the relays? The fuse panel door on my 1998 Jetta is too small and restrictive. I assume I have to remove the entire knee kick panel on the driver's side but I don't want to remove anything that is not necessary.

The black plastic cover that keeps you from poking your toes up into the wiring has to be removed. If I recall there are a pair of phillip headed screws that allow the lower knee protection to be removed by sliding it down, and then the fasteners for the 'toe protection' cover are accessible.

__________________If the quantity of carbonic acid increases in geometric progression,
the augmentation of the temperature will increase nearly in arithmetic progression.
Svante Arrhenius 1896

This is a great thread. Helped me fix some glow plug issues on my 96 B4 that had been going on for some time. The main fuse was pretty much cooked, and when it was REALLY cold, say below 15-20F, the car would start hard with lots of smoke. After replacing that fuse, the cars starts and runs much better. Took me 8 years to find the fuse! Or even know it was there. Thanks for this post!